The Architect of the Capitol surveys the city of Washington that almost was, in an exhibit at the National Building Museum.

It’s hard to imagine a different Capitol sitting atop Jenkins Hill, an elevated site selected by Pierre-Charles L’Enfant, who described it to President George Washington in June 1791 as “a pedestal waiting for a monument.” Washington had directed the three-man board of commissioners brought on to manage the development of the new capital city to hire L’Enfant to design the city and its public buildings. But settling on a design for the Capitol Building wasn’t the most difficult decision of the time. For not only could the Capitol have looked much different than it does today, it might not have been located in the new city named for the father of our country in the first place.

Despite Washington’s influence and stature, locating our nation’s capital in his proposed site near the Potomac River was not a forgone conclusion. Congress had its own ideas of where the capital should reside. Northern members were in favor of sites on the Hudson, Delaware, or Potomac rivers. Southern members liked the idea of two capitals—one on the Potomac and another farther north, such as New York City.

After much debate, arm-twisting, and compromise, the U.S. Senate passed the Residence Act by a vote of 14 to 12. The U.S. House of Representatives followed suit by passing the act by a vote of 31 to 29. Only two scant votes in both the House and the Senate set the course of history. If those two votes had gone the other way in either chamber, I would be reviewing “Unbuilt Philadelphia” or “Unbuilt New York.” But on July 16, 1790, the city of Washington in the District of Columbia was declared the permanent capital of the United States.

“Unbuilt Washington,” on view at the National Building Museum in Washington, D.C., features proposed designs for nearly every notable building on or near the National Mall in Washington today, from the U.S. Capitol Building to the Washington Monument. As Architect of the Capitol, I am responsible for the care and preservation of many of these buildings. Our agency’s portfolio might as well have included several buildings proposed by renowned architects, obscure amateurs, and even U.S. presidents—buildings that were ultimately never built.

The origin of the office of the Architect of the Capitol dates back to the setting of the Capitol’s cornerstone by President George Washington in 1793. Three years earlier, the Residence Act had stipulated that Philadelphia would serve as the temporary capital for 10 years while a new city was built on the northern bank of the Potomac River near Georgetown. While L’Enfant delivered a plan for Washington, despite pressure from the commissioners, by 1791, he had failed to present a plan for the new Capitol. So the leaders of the new democracy took a suitably democratic approach to finding the appropriate design, sponsoring a public competition in March 1792.

While it’s not known exactly how many designs were submitted, at least 13 men are known to have entered, and although some of those proposals are lost to time, 37 drawings still exist today. The evocative drawings exhibited in “Unbuilt Washington” demonstrate how each failed to capture President Washington’s support and imagination—and why they didn’t get built.

Would the Capitol Building still resonate in the world as the definitive symbol of representational democracy if it were built after James Diamond’s design—one that featured what “Unbuilt Washington” curator G. Martin Moeller Jr. has described as a “screaming chicken” atop its unambitious dome? Diamond, an amateur architect, also proposed the same poultry for the President’s House (today, the White House). It’s no surprise that his design wasn’t selected for that building, either.

Then, as is the case now, there were factors and influences beyond aesthetics that went into the decision making. Among them were politics, money (or lack thereof), personal preferences, and war—to name a few.

The exhibit points out that the competition’s winner, William Thornton, adapted his original design—known as the Tortola Scheme—based on the feedback provided by President Washington and others on the prior submissions. Thornton’s first attempt at designing the Capitol looked more like a mansion with wings. Learning of Washington’s predilection for a dome (Washington thought it would give the Capitol “beauty and grandeur”), Thornton scrapped the Tortola Scheme and instead presented a design that featured a low dome. The proposal also incorporated the Neoclassical design that then–Secretary of State Thomas Jefferson preferred. (Thornton, a medical doctor and amateur architect, is credited as the first Architect of the Capitol, because his design for the Capitol was selected.)

More changes were made to Thornton’s plan after a conference was held to discuss the design’s constructability and floor plan. Jefferson—who had submitted his own design for the Capitol, which is on view in “Unbuilt Washington”—lobbied for a three-story House Chamber. Thornton acquiesced, but the concept later proved difficult to build. (After the British burned the Capitol Building in 1814, the design-by-conference approach was scrapped. A new plan for the House Chamber proposed by Benjamin Henry Latrobe, who was responsible for constructing the Capitol’s south wing, was approved in 1815.)

The establishment of the Library of Congress flowed from decisions regarding the Capitol. In 1800, as Congress was preparing to move from Philadelphia to Washington, $5,000 was appropriated to buy books for the use of Congress in its new home. The Library of Congress was thereby created, and it was housed in the new Capitol Building. The Library’s collections continued to grow over the years, even after several fires destroyed portions of it over the ensuing decades. Following much debate and discussion, Congress authorized a design competition in March 1873 for a new library building and appointed a commission to select a plan.

Some of these designs were grandiose, such as Leon Beaver’s palacelike proposal. There were some members of Congress who opposed the construction of a separate library building entirely, and instead advocated for an expansion of the Capitol Building to accommodate the growing collections. A strong proponent of a separate facility, the Librarian of Congress Ainsworth Spofford asked Architect of the Capitol Thomas U. Walter to estimate the cost of enlarging the Capitol for the library. Walter estimated the price tag at about $4 million. Spofford used that estimate to justify funding a separate library, and, in 1886, Smithmeyer & Pelz’s modified design—which was modeled in part on the Paris Opera House—was finally approved.

John L. Smithmeyer and Paul Pelz redesigned their entry many times, submitting Italian Renaissance schemes, Victorian Gothic schemes, and German Renaissance schemes. Their process reminds me of the early American quest to define ourselves. Our newly found freedom and independence was still fresh on our minds; we had yet to determine what style of architecture best defined us as a country. Regarding many of those design decisions, politics, money, and personal preference came into play once again.

The Library of Congress, housed in the Thomas Jefferson Building, was opened to the public on Nov. 1, 1897. On Nov. 25, more than 4,700 visitors toured the new Library during a special Thanksgiving Day open house. The guests were astonished by what they saw. Joseph E. Robinson was so moved, he was compelled to write Spofford: “Not until I stand before the judgment seat of God do I ever expect to see this building transcended.”

Would he have been so moved to write those words by Alexander Esty’s design? “Unbuilt Washington” shows plainly how the course of history might have been altered, had, for example, the Washington Monument been designed as a pyramid, or had our tribute to President Abraham Lincoln been a statue of the man standing atop a ziggurat. Life in Washington would be quite different if the National Mall were bookmarked by elevated highways. But the exhibit also demonstrates how the design history of the capital city affected Americans such as Mr. Robinson.

The early development of our federal city specifically fascinates me. “Unbuilt Washington” goes further: It includes more recent unbuilt work in the Southeast quadrant, Foggy Bottom, and downtown.

Two of these are especially intriguing. The first is the Dolphin America Hotel, designed by architect Doug Michels in 1989. He was rather captivated by dolphins and proposed various projects that would bring humans into closer contact with the aquatic mammals. While we continue to be intrigued by how intelligent dolphins are, we’ll never know if Michels’s idea was a smart one.

The second is the Washington Channel Bridge, proposed by Chloethiel Woodard Smith and Associated Architects in 1966 to connect the Southwest Waterfront to East Potomac Park. Imagined as Washington’s version of Florence, Italy’s Ponte Vecchio, this pedestrian bridge would have featured shops and restaurants, potentially altering the look and feel of these neighborhoods so dramatically that they might be unrecognizable today. I find myself a little disappointed that this bold idea was never built.

Woven throughout the exhibit are the “Nationals.” (No, not the baseball team that led the National League East in April.) The exhibit captures our country’s efforts to define itself through so-called “national” monuments, memorials, and buildings. The exhibit surveys designs for the National Mall, the National Cathedral, a National Museum, a National University, a National Aquarium—even a National Parthenon.

Despite the fact that the United States never built a National Parthenon or a dolphin center, I can’t help but think that somehow we got it right. Yes, we occasionally veered off course, and the history of Washington, D.C., is punctuated with corrections, such as the McMillan Plan (1901) and the National Capital Planning Commission’s Extending the Legacy Plan (1997). But even through the ebb and flow of public support, and notwithstanding the influence of politicians, politics, war, and money (or lack thereof), the nation has generally arrived at the right answers and the best design solutions for its capital.

I get a lump of pride in my throat whenever I fly back to Washington after some time away. Is there anything more majestic than seeing the Washington Monument appear outside your airplane window and having your eye wander up the great, green expanse of the National Mall to the gleaming dome of our Capitol Building? One of the best features of “Unbuilt Washington” is that it is set in Washington—the iconic, monumental, and inspirational Capital City.